The present invention relates to an exposure time control circuit of a camera for automatically controlling the exposure time in dependence upon the brightness of the object field and, in addition thereto, for automatically controlling other exposure variables.
Camera control circuits utilizing a monolithic IC having a parabolic characteristic are known in the art. In general, the IC includes signal input circuitry in the form of a transistorized differential amplifier and therefore the IC possesses very good voltage and temperature characteristics. To one input terminal of the IC is applied a timing signal for controlling the exposure time and to another input terminal is applied a standard or reference voltage signal, both signals being fed to the differential amplifier to effect control of the exposure timing.
The drawback of this type control circuit is that due to the base-emitter voltage characteristic of the transistors which constitute the differential amplifier, it is not possible to vary the level of the reference voltage except within a very narrow range. This prevents use of these input terminals as inputs for introducing other exposure information variables, such as input signals representative of the F-value, film sensitivity, etc., whose reference voltage level must be capable of varying over a wide range. Moreover, in case of connecting the polarity of the electric source battery reversely, there is a risk of damaging the IC by excessive current flow owing to the use of the monolithic IC.
The exposure time control circuit of the invention enables various exposure information variables, in addition to that representative of the brightness of the object field, to be applied as inputs to the IC. In addition, the control circuit of the invention prevents break-down of the IC due to reverse connection of the electric source battery.